1. Field of the Invention
The present invention relates generally to regulated power supplies, and in particular to methods and apparatus for dissipating power in a monolithic linear voltage regulator.
2. Description of the Related Art
Linear regulators are used to generate a constant output voltage which is, within limits, independent of load current and input voltage. One such regulator is a linear buck regulator, wherein the regulated output voltage is less than the input voltage. With reference to FIG. 1, one type of buck regulator is a shunt regulator 100. An input voltage is provided at a voltage input 102 which is connected to one side of a resistor 104. The other side of resistor 104 is coupled to an output voltage node 107, to a load 106 and to one side of a zener diode 108. The zener diode 108 is connected in parallel with load 106 and operates as a non-linear resistance to regulate the potential across load 106 by diverting that portion of the current flowing through resistor 104 which is not provided to load 106. Resistor 104 in turn limits the amount of current drawn by both zener diode 108 and load 106.
Shunt regulator 100 has many advantages. First, it is simple and inexpensive. Second, a discrete through hole (i.e., stud mounted) zener diode or a surface mount zener diode and series resistor can dissipate power (heat) more efficiently than a monolithic arrangement due to thermal impedance between the device (diode and/or resistor) and ambient. However, shunt regulator 100 also has disadvantages. For example, shunt regulator 100 provides inferior line and load regulation when compared to a series regulator. In addition, the current flowing through series resistor 104 directly affects the dropout voltage, that is, the difference in potential from voltage input 102 at node 107 at which regulation ceases.
With reference now to FIG. 2, a series regulator circuit 200 includes a voltage input 202 which is connected to a power source having an unregulated voltage greater than that desired across a load 204. Voltage input 202 is connected to a pass device 206, which typically is a transistor, either integrated within a monolithic regulator's die or a separate discrete device. With this approach, the majority of power supplied by the power source 202 and not provided to load 204 is dissipated in pass device 206. Pass device 206 is further connected at an output voltage node 207 to both load 204 and a resistive divider network 208, which divider network consists of a pair of resistors 210 and 212. The junction of resistors 210 and 212 provides to an inverting input 214 of an error amplifier 216 a known proportion, (R.sub.212 /R.sub.210 +R.sub.212)), of the potential across load 204, Vout. A voltage reference 220 provides a constant voltage at a non-inverting input 222 of error amplifier 216. An output 224 of error amplifier 216 is coupled to the pass device (typically a base for a bi-polar transistor, or a gate for a MOS transistor or a field effect transistor). In operation, the known ratio of the voltage across load 204 is provided via divider network 208 and subtracted from the potential of voltage reference 220 by error amplifier 216. The output 224 in turn, directly or indirectly, controls the impedance between nodes 202 and 207 of pass device 206. Stated differently, pass device 206 operates as a variable resistor in series with load 204. As the potential at voltage input 202 changes, and/or as the current drawn by load 204 changes, the feedback provided through error amplifier 216 varies the impedance of pass device 206 from node 202 to node 207 to thereby maintain the desired regulated voltage, Vout, across load 204. Given that the power dissipation of pass device 206 is essentially equal to the product of the current flowing through pass device 206 and the voltage drop across pass device 206, for the same power dissipation, a pass device within a monolithic regulator is normally more expensive than either a discrete through hole pass device or a surface mount pass device because of related packaging costs and heat conduction requirements (thermal impedance from the die to ambient). For this reason, external resistors have been used to dissipate a portion of the power in a monolithic series regulator in order to reduce cost.
FIG. 3 illustrates a regulator circuit 300 which uses a series resistor approach for reducing power dissipation of a pass device. In further detail, an external resistor 302 is connected between a pass device 304 and a load 306 from a power source having an unregulated voltage greater than that desired at an output voltage node 307. As with the series regulator circuit 200 of FIG. 2, a voltage input 308 provides current into pass device 304. A resistive voltage divider 310 consists of a resistor 312 and a resistor 314 which together provide a known proportion (R.sub.314 /R.sub.312 +R.sub.314)) of the potential at node 307 (and across load 306), Vout, to an inverting input 316 of an error amplifier 318. A voltage reference 320 provides a constant potential to a non-inverting input 322 of error amplifier 318. An output 324 of error amplifier 318, in response to the potential provided to inverting input 316, provides a yawing potential to pass device 304 to thereby vary the impedance of pass device 304 from voltage input 308 to node 303.
In operation, dissipated power is diverted from the pass device 304 in which regulator circuit 300 resides to external resistor 302. However, as with shunt regulator 100 of FIG. 1, resistor 302 increases the regulator's dropout voltage because the current drawn by load 306 also flows through resistor 302. If there is a varying input voltage at voltage input 308 the value of resistor 302 must be selected so that the additional IR drop (the voltage drop equal to the product of the current through a resistor and the value of the resistor) of resistor 302 does not cause the pass device 304 to saturate at the lowest input voltage, Vin.sub.13 low, with worst case high load current, I.sub.-- max. Ignoring the saturation voltage of the pass device 304, the value of resistor 302, S.sub.-- Rext, can be expressed as:
S.sub.-- Rext=(Vin.sub.-- low-Vout)/I.sub.-- max. PA1 Vin.sub.-- low=9 volts, PA1 Vout=5 volts, PA1 I.sub.-- max=0.125 amperes, PA1 S.sub.-- Rext=32 ohms. PA1 P.sub.-- Rext=(Vin.sub.-- high-Vout)I.sub.-- min. PA1 Vin.sub.-- high=16 volts, PA1 Vout=5 volts, and PA1 I.sub.-- min=0.083 amperes, PA1 P.sub.-- Rext=132 ohms.
Applying this equation to, for example, an automotive environment where the battery/alternator system voltage can vary from 9 volts to 16 volts D.C.), assuming that the amount of current provided to load 306, at a potential of 5 volts, varies from 0.083 to 0.125 amperes:
Thus,
Transients, however, appearing at voltage input 308 must also be accounted for when selecting the value, S.sub.-- Rext, of resistor 302. In addition, when load 306 is dynamic, the current through load 306 can momentarily exceed I.sub.-- max. Thus, by accounting for these factors, the value, S.sub.-- Rext, of resistor 302 must therefore be deceased, thereby decreasing the effectiveness of using a series approach.
FIG. 4 illustrates another regulator circuit 400, which utilizes a resistor 402 in parallel with a pass device 404. In further detail, regulator circuit 400 includes a voltage input 406 connected to the junction of resistor 402 and pass device 404. Pass device 404 and resistor 402 are connect to an output voltage node 407. A load 408 is also connected to output voltage node 407. The potential at output node 407, Vout, is divided by a divider network 410 which consists of a pair of resistors 412 and 414. The junction of resistors 412 and 414 is connected to an inverting input 416 of an error amplifier 418. A voltage reference 420 is connected to a non-inverting input of error amplifier 418. An output 424 of error amplifier 418 is coupled to a control element (such as a base of a bipolar transistor or a gate of a MOSFET) of pass device 404.
In operation, a portion of the current provided to load 408 flows through resistor 402, the amount of current flowing through resistor 402 being a function of the difference between the potential at voltage input 406, Vin, and the potential across load 408, Vout. The remainder of the load current flows to load 408 through pass device 404. If Vin at voltage input 406 goes too high or the current through load 408 goes too low, pass device 404 turns off and all of the load current then flows through resistor 402, resulting in a cessation of regulation. Therefore, in order to maintain regulation, the value of resistor 402 must be selected based upon the minimum load current, I.sub.-- min, and the maximum potential at voltage input 406, Vin.sub.-- high. Not accounting for transients, the desired value of resistor 402, P.sub.-- Rext, can be expressed as:
If
Then,
As with the regulator circuit 300 which utilizes resistor 302, transients at voltage input 406 and in the load current should also be accounted for when calculating P.sub.-- Rext. Load currents can momentarily go below I.sub.-- min with dynamic loads, and transients above Vin.sub.-- high may appear due to changing loads connected in parallel with voltage input 406. Thus, when transients are accounted for, the value of resistor 402, P.sub.-- Rext, must be increased, which in turn decreases the effectiveness of using resistor 402 in dissipating power. One major advantage of the resistor approach of FIG. 4 over the series resistor approach of FIG. 3 is that, with the parallel approach, the regulator's dropout is basically a function of the pass device 404.
With both the series resistor approach of FIG. 3 and the resistor approach of FIG. 4, their effectiveness as regulators decreases as the range of Vin increases and as the range of the load current increases. The regulator circuit 300 of FIG. 3 more effectively transfers power dissipation to an external resistor with high values of input voltage, Vin, and small load currents. The opposite is true with respect to the regulator circuit 400 of FIG. 4.
Thus, it would be desireable to provide a voltage regulator which does not suffer from the disadvantages of either the series resistor approach or the parallel resistor approach, yet more effectively dissipates power. For a given range of load currents and range of input voltages, it would also be desireable to provide a voltage regulator which dissipates less power in the pass device than the above described circuits.